We have previously shown in three separate groups of patients with Seasonal Affective Disorder (SAD) that extending the photoperiod in the morning and evening with bright full-spectrum light leads to a significant antidepressant response whereas equivalent treatment with dim light does not. We have shown that this antidepressant effect is not the result of sleep deprivation alone. However, since in the past there has always been some element of sleep deprivation involved in both active and control treatments, we could not rule out sleep deprivation as a contributing factor to the antidepressant response. Sleep deprivation occurred largely as a result of patients having to wake early in the morning for light treatment. This past year we explored the effects of light treatment given in the evening only without any element of sleep deprivation. This also enabled us to investigate whether the early morning hours are critical for the antidepressant response to light, as has been suggested to be the case for sleep deprivation. In nine patients we studied the antidepressant effects of five hours of bright (2500 lux) light given after dusk and compared the results to an equivalent amount of dim (300 lux) light given at the same time. Although the bright light appeared to be somewhat effective, there was no significant difference in the degree of mood change when effects of bright and dim light were compared with each other. Further studies will continue to explore the formal properties of the antidepressant effects of light in SAD. We are especially curious to determine whether, given equal amounts of bright light, the timing of the administration of the light will affect outcome.